Effects of retention level and fire on retention tree dynamics in boreal forests

• Higher retention levels can maintain post-harvest continuity of dead wood.
• Fire increases mortality of retention trees.
• Fire increases diversity of dead and living trees in retention forestry.
• The continuity of habitat substrates will be lost if retention level is too low.

Retention forestry has been used for over 20 years to reduce the unfavorable impacts of intensive forest management on biodiversity. The assumed positive effects of retention trees, however, depend on the dynamics of trees in providing substrates or structures for forest-dwelling organisms. In 2000 an experimental study was established to investigate the effects of different retention levels (10 m3 ha−1 and 50 m3 ha−1) and fire on tree dynamics. In total, 2758 individually marked, initially living, retention trees were followed on 12 sites in eastern Finland over 10 post-harvest years. Over half (59%) of the total volume of the retention trees died during these initial 10 years, and burning resulted in much higher mortality (84% vs. 34% on unburned sites). At lower retention levels, retention trees did not provide continuity of habitat substrates since all trees died quickly. Fire shortened the tree availability, due to increased tree mortality. However, in higher retention levels, burned areas maintained diverse deadwood substrates for an extended period. Our study proved that tree retention can maintain the continuity of dead wood over early successional stages, if the level of retention is high enough. Fire, combined with higher retention level, created diverse assemblages of dead and living trees. At lower retention levels, however, the effect of fire can be too severe for maintaining living trees or continuity of diverse dead wood.